serials: API for safe deleting subscriptions

serials: API for safe deleting subscriptions

and distributions and streams.

open-ils.serial.subscription.safe_delete
open-ils.serial.distribution.safe_delete
open-ils.serial.stream.safe_delete
open-ils.serial.subscription.safe_delete.dry_run
open-ils.serial.distribution.safe_delete.dry_run
open-ils.serial.stream.safe_delete.dry_run

Won't delete if there are attached serial.item's with a status other
than Expected, or if there are any attached non-deleted serial.unit's.

dry_run variants behave exactly the same except they don't actually
delete anything.

Signed-off-by: Jason Etheridge <jason@equinoxinitiative.org>